1. The Field of the Invention
The invention generally relates to tunnel junctions. More specifically, the invention relates to using a super lattice structure in conjunction with a tunnel junction to provide an improved contact for multiple components.
2. Description of the Related Art
Tunnel junctions are used in various applications to provide a common connection between two components. One application of tunnel junctions is in the design of components made at least in part from III-V (three-five) semiconductor materials. In such applications it can be advantageous to have a low resistance tunnel junction. Often, in order to obtain a low resistance, the tunnel junction must be heavily doped to reduce the bandgap. In some instances a tunnel junction made from a combination of III-V semiconductor materials would have the desired resistance properties, but it has not been possible to use this combination of III-V semiconductor materials for various reasons. One reason why a combination of III-V semiconductor material cannot be used is because the III-V semiconductor materials have a mismatched lattice parameter resulting in a critical thickness that is too thin to be used without the tunnel junction experiencing a dislocation and subsequent cracking.
There are several III-V semiconductor materials that would otherwise be used to create tunnel junctions with desirable low resistance properties if not for their undesirable lattice parameters. One example of a III-V semiconductor with such properties is Indium (In). In can be combined with other III-V semiconductor materials, such as Gallium (Ga) and Arsenide (As). Two constraints, however, that limit the relative proportion of In to Ga and As are the necessary thickness of a tunnel junction and the critical thickness of the material due to lattice mismatched parameters. The critical thickness of the material, however, is a function of the proportion of the In relative to the GaAs in the material. As illustrated in FIG. 1, and is known to one of ordinary skill in the art, as the relative proportion of the In increases, the critical thickness of the InGaAs material becomes thinner. Similarly, as the relative proportion of In increases, the resistance of the resulting tunnel junction decreases. In other words, a relatively high proportion of In is desirable to reduce the resistance of the tunnel junction, but the thickness of a tunnel junction is limited because of the critical thickness of InGaAs due to the mismatched lattice parameter of the materials that can result in dislocation. For example, it may be advantageous to use about 15% In, which can result in a critical thickness of InGaAs at about 150 angstroms. This is typically too thin for a tunnel junction in applications where the tunnel junction should be equal to or greater than 150 angstroms, for example where a tunnel junction with a thickness of about 500 angstroms is.
Components made from III-V semiconductor materials include optoelectronic devices, such as lasers, light emitting diodes (LEDs), and photodiodes. Lasers have become useful devices with applications ranging from simple laser pointers that output a laser beam for directing attention, to high-speed modulated lasers useful for transmitting high-speed digital data over long distances of optical fiber. Several different types of lasers exist and find usefulness in applications at the present time. One type of laser is the edge emitting laser, which can be formed at least in part by cleaving a diode from a semiconductor wafer. Cleaving a diode from a semiconductor wafer forms facets creating reflective surfaces that form a laser cavity defined by the edges of the laser diode. Reflective and antireflective coatings can be applied to the facets of edge emitter lasers, and edge emitting lasers can be designed to emit a laser beam more strongly from one of the edges than the other edges. However, some laser energy is typically emitted at the other edges.
A second type of laser is known as a vertical cavity surface emitting laser (VCSEL). A VCSEL can include semiconductor active regions, distributed Bragg reflector (DBR) mirrors, current confinement structures, substrates, and contacts. A VCSEL can be formed in part by forming a first mirror from DBR semiconductor layers. The DBR layers can alternate high and low refractive indices so as to create the mirror effect. An active layer can then be formed on the first mirror. A second mirror can be formed on the active layer using more DBR semiconductor layers. Thus the VCSEL laser cavity can be defined by top and bottom mirrors that cause a laser beam to be emitted from the surface of the laser. Laser diodes typically operate using a forward bias. To forward bias a laser diode, a voltage can be applied to the anode and a lower voltage or ground can be connected to the cathode.
In some simple applications, lasers may be operated open loop (i.e., the lasers do not require feedback, or can operate satisfactorily without feedback). In other applications, it may be very important to precisely gauge the amount of actual output power emitted by the laser while it is operating. For example, in communications applications it may be useful to know the actual output power of the laser such that the output power of a laser may be adjusted to comply with various standards or other requirements.
Many applications use a laser and a photodiode or other photosensitive device to control the output of the laser. An appropriately placed photodiode can be used as one element in the feedback circuit for controlling the laser. Various challenges exist, however, when implementing a laser diode and photodiode together. While the laser diode and photodiode share a similar construction and composition, they have generally been implemented as separate devices. This allows a single power supply to be used for both biasing the laser diode and photodiode which are biased using opposite polarities. Using two discrete components, however, results in an increase of cost.
Attempts have been made to integrate the laser diode and photodiode monolithically on a single wafer substrate. However, as alluded to above, this may require the use of two power supplies such as in the case when the laser diode and photodiode share a common cathode or anode. Additionally, the photodiode may be placed on top of the VCSEL or within a mirror that is part of the VCSEL. This however has the unfortunate drawback of causing the photodiode to become a part of the optics, particularly the mirror, of the laser thus altering the optical characteristics of the laser. Therefore, what would be advantageous are improved methods and apparatuses for providing a common connection to multiple component using a tunnel junction.